What version of Studio are you running? When i use my 72 studio on our 68 server I have issue where sometimes (randomly) the variables disappear. ie: they will disappear for one line breakpoint and then appear again for the next. Making it very tricky and annoying to debug! The way I get around it is by inserting lots of dummy lines in the code and setting a bunch of extra breakpoints.
ie:
debugStopHere := 'y';
debugStopHere := 'y';
debugStopHere := 'y';
Just for curiousity sake you can try entering a few fake lines below your select and put a breakpoint on each line then step through and see if it appears for any of them? At least then we will know that it is capable of appearing at least some time. The good news is I have another rev 82 database with 82 studio and I do not experience this same problem. I definitely found rev 68 and 72 very buggy with the debugger. ie: sometimes I also set the break point on a certain line and it inserts the breakpoint on the line after which is also very annoying.
-Patrick